Select profile image from Asana correctly

Summary: Ref T2852. The `picture` field is actually a dictionary of options. Choose the largest photo.

Test Plan: Logged in with Asana and saw my profile image populate correctly.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2852

Differential Revision: https://secure.phabricator.com/D6431